Saturation-induced coherence loss in coherent backscattering of light

Abstract

We use coherent backscattering (CBS) of light by cold Strontium atoms to study the mutual coherence of light waves in the multiple scattering regime. As the probe light intensity is increased, the atomic optical transition starts to be saturated. Nonlinearities and inelastic scattering then occur. In our experiment, we observe a strongly reduced enhancement factor of the coherent backscattering cone when the intensity of the probe laser is increased, indicating a partial loss of coherence in multiple scattering.PACS numbers: 42.25.Fx, 32.80.Pj Wave coherence in the multiple scattering regime is a key ingredient to reveal the impact of interference on wave transport in strongly scattering media, with special interest in photonic crystals [1], random lasers [2], strong and weak localization [3,4]. It has been shown that wave coherence has some robust features which survive the spatial configuration average. A clean illustration is given by the coherent backscattering phenomenon (CBS) [5,6], a random two-wave zero path length interferometer. CBS manifests itself as an enhanced diffuse reflection peak in the backscattering direction. This signal is related to the Fourier transform of the configurationaveraged two-field correlation function (the mutual coherence) at two space-time points at the surface of the medium [7]. Hence the enhancement factor α (the peak to background ratio) provides a simple measure of spatial coherence properties of the disordered system after spatial averaging. For perfect contrast, α takes its maximal value of two, a symmetry property bearing on reciprocity [8]. In recent experiments, we have studied CBS in the elastic scattering regime with cold atomic vapours exposed to low intensity quasi-resonant monochromatic light. We have evidenced a loss of contrast due to the internal structure of atoms [9, 10, 11] and a full contrast restoration when non-degenerate atoms are used [12].Studying optical wave transport and localization effects with cold atoms offer several advantages. Indeed, they act as ideal point-like scatterers, where the scattering matrix can be fully described with ab initio calculations and no adjustable parameters. Moreover, the presence of sharp resonances results in large scattering cross-sections, easily tunable by few orders of magnitude, and large associated time scales, making resonant scattering systems different from non-resonant multiple scattering studied so far. If strong driving fields are used, atoms exhibit unusual scattering properties. First the atomic susceptibility shows up a dependence on the local field intensity. This non linearity alters both scattering (nonlinear reduction of the scattering cross-section) and propagation (generation of a nonlinear refractive index for the effective medium). Second, in addition to the usual elastic component, atoms radiate an inelastic spectrum component. For resonantly driven atoms with a non-degenerate groundstate, a characteristic frequency width of this spectrum is Γ (inverse of the excited-state lifetime) [13]. ...

Spectrum of coherently backscattered light from two atoms

“…Experimental studies on cold Sr atoms revealed a rapid decrease of the CBS interference contrast with increasing intensity of the injected laser field, as a consequence of the saturation of the laser-driven atomic transition [8]. A much weaker sensitivity of the quality of the CBS signal was observed for Rb atoms [9].…”

“…We address a regime of multiple scattering which is relatively simple from the theoretical point of view: scattering by an optically thin atomic medium, where double scattering provides the dominant contribution to the CBS signal [10,15]. It is in this double scattering regime where the first observation of a CBS reduction due to the saturation of atomic dipole transitions was reported [8].…”

“…1(b)], precisely as in the Sr experiment [8]. The laser intensity is encoded in the saturation parameter s = Ω 2 /2(γ 2 + δ 2 ), where Ω is the Rabi frequency, γ is half the spontaneous decay rate of the atomic excited states, and δ = ω L − ω 0 is the laser-atom detuning.…”

Double scattering of intense laser light by two atoms

“…On the other hand, for degenerate (e.g., in rubidium atoms) and non-degenerate (e.g. in strontium atoms [8]) transitions alike, increasing the intensity of the incident laser field leads to atomic saturation accompanied by nonlinear (multi-photon) inelastic scattering processes [9] reducing the interference contrast in CBS [10].…”

“…While a thorough theory of CBS of light from degenerate atoms in the regime of elastic scattering already exists [11,12], a theory of CBS from saturated atoms is still in a rudimentary state, several years after the "saturation" experiments with strontium [10] and rubidium [13] atoms. The main obstacle for developing such a theory is that CBS becomes a multi-wave interference effect in the nonlinear scattering regime [14,15], with more than two interfering amplitudes.…”